Conclusions 
Pollen morphology of Quercus pollen in the United States is rarely studied. Sizes between Quercus pollen of 3 oak sections, of 5 phylogenetic groups, and of 23 species were applied LM and SEM morphological measurements to investigate classification and differentiation between phylogenetic lineages of Quercus native to California.
Jarvis et al.’s (1992) study of oak morphology suggests that both polar axis and equatorial diameter of evergreen oak pollen are smaller than that of deciduous oak pollen in southwestern Sichuan Province, China . The observation was not found in this study. Even so, deciduous oak pollen tends to perform shorter furrows than evergreen oak pollen does. New World Quercus section, Protobalanus , has the smallest average size and variation in three studied sections. This group is unique by its smooth pollen surface and uniformed dense scabratae. The higher similarity of sizes between Quercus and Lobatae sections coincide with the closer phylogenetic distance of these two sections than them and Protobalanus . Pollen grains of studied species in Quercus and Lobatae sections were classified into “(micro) verrucate” type, while that in Protobalanus was classified into “rodlike masked” type. We manually separated the ultrastructure features into 3 types. Type 1 and type 2 can be matched to (micro) verrucate category in Denk’s and Grimm’s classification (Denk and Grimm, 2009). Type 2 pollen grains have ultraelements on each “(micro) verrucate” elements which type 1 pollen grains do not have. Type 3 can be matched to rodlike masked category.  The length of polar view character shows phylogenetic signal in 20 Quercus species. The distance between 2 furrows in Lobatae sections, Q. agrifolia, Q. kelloggii, Q. parvula, and Q. wislizeni, also has phylogenetic signal. Further investigations will be focused on details of surface ultrastructure and trait-environment relationships.